             ;; so it shouldn't happen here.
             acked-blocks (filter #(= ack-id (::specs/message-id %))
                                  un-acked-blocks)
-            flagged (-> (reduce flow-control/update-statistics
-                                ;; Remove parent->buffer.
-                                ;; It's been parsed into packet
-                                (update state ::specs/incoming
-                                        dissoc
-                                       ::specs/parent->buffer)
-                                acked-blocks)
-                        ;; That takes us down to line 544
-                        ;; It seems more than a bit silly to calculate flag-acked-others!
-                        ;; if the incoming message is a pure ACK (i.e. message ID 0).
-                        ;; Leave it be for now, just to try to stay in sync with reference
-                        ;; implementation, but this smells.
-                        (flag-acked-others! packet))
+            ;; That takes us down to line 544
+            ;; It seems more than a bit silly to calculate flag-acked-others!
+            ;; if the incoming message is a pure ACK (i.e. message ID 0).
+            ;; That seeming silliness is completely correct: this
+            ;; is the entire point behind a pure ACK.
+            acked-gaps (flag-acked-others! packet)
+            flagged (reduce flow-control/update-statistics
+                            ;; Remove parent->buffer.
+                            ;; It's been parsed into packet
+                            (update acked-gaps
+                                    ::specs/incoming
+                                    dissoc
+                                    ::specs/parent->buffer)
+                            acked-blocks)
             extracted (extract-message! flagged packet)]
         (log/debug (str message-loop-name
                         ": handle-comprehensible message/extracted:\n"

                     :packet ::specs/packet)
        :ret ::start-stop-details)
(defn calculate-start-stop-bytes
  "calculate start/stop bytes (lines 562-574)"
  [{{:keys [::specs/receive-bytes
            ::specs/receive-written]
     :as incoming} ::specs/incoming
    :keys [::specs/message-loop-name]
    :as state}
   ;; Q: Isn't this a byte array now?
   {^ByteBuf incoming-buf ::specs/buf
    D ::specs/size-and-flags
    start-byte ::specs/start-byte

                   [::specs/ack-gap-4->5 ::specs/ack-length-5] ; 30-32
                   [::specs/ack-gap-5->6 ::specs/ack-length-6]])] ; 34-36
    (log/debug (str message-loop-name
                    ": ACK'ing with Gaps: " (into [] gaps)
                    "\nState: " state))
    (->
     (reduce (fn [{:keys [::stop-byte]
